The present invention relates to chemical analysis. More particularly, the invention relates to a flow cell for use with flow-based chemiluminescence measurements.
Chemiluminescence is the generation of light from chemical reactions. Chemiluminescence processes have attracted mankind's attention for centuries. Aristotle wrote the first known report on the phenomenon when he noted that weak light was emitted by dead fish and fungi. The term chemiluminescence was first defined by Wiedemann in 1888 as light emitted from chemical reactions.
Many chemiluminescence reactions are now well known. Early studies of chemiluminescence focused primarily on the chemistry and mechanisms of chemiluminescence reactions. In the early 1960's, analytical applications of chemiluminescence reactions began to appear in the literature. Since then, chemiluminescence analytical methods have grown substantially, due to the advantages of low detection limits, wide linear dynamic ranges, and rapid response.
The early analytical applications involved manual techniques for mixing reagent and sample, and measuring the light emitted. In 1975, Ruzicka and Hansen introduced Flow Injection Analysis, which provided a new tool for performing chemiluminescence analyses. With Flow Injection Analysis, reagent and sample can be automatically mixed rapidly and reproducibly in a flowing stream, in close proximity to a chemiluminescence detector. Flow cell designs which caused reagent and sample to merge close to the light-sensing detector allowed rapid chemiluminescence chemistry to be monitored. Critical to the success of this mode of operation is rapid and efficient mixing of the components. This automation made chemiluminescence an even more attractive analytical technique.
A typical Flow Injection Analysis chemiluminescence configuration is shown in FIG. 1.
Containers of a carrier liquid 2 and of a reagent 4 are connected to a pump 8, and a sample 6 is injected into a flowing stream 2a of the carrier liquid 2. A reagent stream 4a merges at a T connection 9 with the stream 2a, which now contains the sample 6. The emerging stream 9a, now containing carrier liquid 2, reagent 4, and sample 6, flows for mixing through a mixing coil 14 into a flow cell 10, and light emitted by chemiluminescence is detected and its intensity measured by a detector 12.
In 1990 Ruzicka and Marshall introduced Sequential Injection Analysis. This method is a variant of Flow Injection Analysis which offers some important advantages. Whereas with Flow Injection Analysis the sample is injected into a flowing carrier stream, with Sequential Injection Analysis adjacent sample and reagent zones are aspirated into a holding coil, and then the flow is reversed to transport the zones to the detector. Mixing and chemical reaction between the zones occur during transport. Means have been developed to promote rapid radial mixing while minimizing axial dispersion. These means typically include pumping the zone stack through a torturous path involving rapid changes in direction of flow. Sequential Injection Analysis can be performed with simpler apparatus, and uses considerably less reagent as compared to Flow Injection Analysis.
A typical configuration of apparatus for Sequential Injection Analysis is shown in FIG. 2.
Containers of a carrier liquid 2, a reagent 4, and a sample 6 are connected to a bidirectional pump 8 through a selection valve 16 having a common port 16a, and an outlet port 18 connected to a flow cell 10. Mixing of the sample 6 and the reagent 4 occurs in a holding coil 15. Intensity of chemiluminescence is detected and measured by a detector 12.
Typically, in Flow Injection Analysis and Sequential Injection Analysis chemiluminescence systems, as well as in post-column chemiluminescence derivatization with liquid chromatography, a length of coiled tubing is used as the flow cell. A schematic representation of such a cell for Flow Injection Analysis is shown in FIG. 3. Streams 2a of sample and 4a of reagent merge in a T-connection 9 external of the coil 9a, which initiates mixing, and the reaction mixture flows through the coil 9a, emitting light which is captured by a detector 12. After flowing through the coil 9a, the mixture is discharged to waste via a tube 18.
While this type of simple flow-through cell works well, it has a number of shortcomings that impact its performance; viz.:                (a) Only a coil configuration is practical. Other flow paths, such as reversing turns which enhance mixing, are not possible or easily achieved. While mixing is initiated in the T-connection 9, it is not complete, and further but still incomplete mixing occurs during transit of the reacting zones through the coil 9a.         (b) Mixing is initiated external to the coil 9a where no light is captured; so for very fast reactions, especially with rapid decay of the chemiluminescence, some of the emitted light is lost prior to detection.        (c) Commonly-used polymeric tubing is not completely transparent; it is generally translucent. Hence, some light is lost in the tubing walls.        (d) Tubing has a curved wall, which causes light loss due to reflection, compared with a flat wall.        (e) The internal diameter of the channel is limited by the availability of tubing sizes.        
The present invention provides a flow cell which eliminates all of the limitations of the coiled tube, and is thus more efficient at generating and transmitting light produced by chemiluminescence.